In recent years, recording and reproducing apparatus and the like using discs to be rotated experience an increase in a memory capacity and an increase in a transfer rate for data. Thus, bearings used for such recording and reproducing apparatus are required to have high performance and high reliability to constantly rotate a disc load with a high accuracy. Accordingly, hydrodynamic bearings suitable for high-speed rotation are used for such rotary devices.
The hydrodynamic bearing type rotary device has a lubricant such as oil between a shaft and a sleeve, and generates a pumping pressure by hydrodynamic generating grooves during rotation. Thus, the shaft rotates in a non-contact state with respect to the sleeve. Since no mechanical friction is generated, the hydrodynamic bearing type rotary device is suitable for high-speed rotation.
Hereinafter, an example of conventional hydrodynamic bearing type rotary devices will be described with reference to FIGS. 11 through 13.
As shown in FIG. 11, a conventional hydrodynamic bearing type rotary device includes a sleeve 21, a shaft 22, a stopper 23, a bottom plate 24, oil 25, a hub 27, a base plate 28, a rotor magnet 29, a stator 30, and a disc 31.
The shaft 22 is integrated with the hub 27 by press fitting, adhering, press-fit adhering, or the like. The shaft 22 is inserted into a bearing hole 21A of the sleeve 21 so as to be rotatable. The stopper 23 is fixed to the shaft 22 by a screw or press fitting, and is accommodated within a step portion 21C of the sleeve 21. On at least one of an inner peripheral surface of the sleeve 21 and an outer peripheral surface of the shaft 22, radial hydrodynamic generating grooves 21B are formed to form a radial bearing surface. On a surface of the sleeve 21 facing the hub 27 on the rotor side, thrust hydrodynamic generating grooves 21D having a spiral pattern as shown in FIG. 12 are formed to form a thrust bearing surface. The bottom plate 24 shown in FIG. 11 is fixed to the sleeve 21. On an outer peripheral surface of the sleeve 21, a tapered portion 21E is provided. A seal portion 32 is provided between the tapered portion 21E and a circular protrusion 27A of the hub 27. A lubricant such as the oil 25 is sealed in the bearing cavity, and a gas-liquid boundary surface of the lubricant is formed near the seal portion 32. The sleeve 21 is processed so as to have a vent hole 21F which helps discharging air.
To the base plate 28, the sleeve 21 is fixed. The stator 30 is also fixed to the base plate 28 so as to oppose the rotor magnet 29. Magnetic centers of the rotor magnet 29 and the stator 30 in an axial direction are largely shifted, and thus, the rotor magnet can generate an attraction force in a direction indicated by arrow M in the figure. To the hub 27, the rotor magnet 29 and the disc 31 are fixed.
Operations of the conventional hydrodynamic bearing type rotary device having the above-described structure are as follow. In the conventional hydrodynamic bearing type rotary device shown in FIG. 11, when an electric current is supplied to the stator 30, a rotary magnetic field is generated, and a rotary force is applied to the rotor magnet 29. Thus, the rotor magnet 29 starts to rotate with the hub 27, the shaft 22, the stopper 23, and the disc 31. When these members rotate, the radial hydrodynamic generating grooves 21B gather the oil 25 filled in the radial gap, and a pumping pressure is generated between the shaft 22 and the sleeve 21. The thrust hydrodynamic generating grooves 21D gather the oil 25, and a pumping pressure is generated between the hub 27 and the sleeve 21. The shaft 22 and hub 27 float in a direction opposing the attraction force of the rotor magnet 29 which is indicated by arrow M in the figure, and starts to rotate in a non-contact state.
If there is air in the bearing cavity, the air probably passes through the vent hole 21F and is discharged from the gas-liquid boundary surface of the seal portion 32. The term “probably” is used because the air entered in bearing may go to the gas-liquid boundary surface or may go back to the bearing, and not necessarily discharged to the gas-liquid boundary surface.
As described above, the shaft 22 can rotate in a non-contact state with respect to the sleeve 21. With a magnetic head or an optical head (not shown), data can be recorded/reproduced to/from a rotating disc 31.